Polyester film

ABSTRACT

A polyester film which is provided with a permanent, antistatic coating on one surface, which coating is obtained by treating the biaxially stretch-oriented and heat-set film with an aqueous dispersion, containing at least one copolyester containing sulfonate groups, and at least one salt derived from an amino acid or a protein hydrolysate and having a diazotype reproduction layer on the opposite surface of the film.

This application is a division of application Ser. No. 133,486, filedDec. 15, 1987, now U.S. Pat. No. 4,876,155.

BACKGROUND OF THE INVENTION

The present invention relates to a biaxially stretch-oriented andheat-set polyester film which on at least one surface is provided with acoating in which a copolyester that includes sulfonate groups iscontained.

The manufacture of copolyesters containing sulfonate groups and theiruse as constituents of coatings for plastic films are known (U.S. Pat.No. 3,563,942). They can be applied in the form of aqueous solutions ordispersions and are employed for layer supports or adhesion-promotingintermediate layers (German Patent No. 28 13 014). The copolyestercoatings per se do not have an antistatic effect. They may be applied tothe finished plastic film or during film manufacture (U.S. Pat. No.4,476,189). Apart from their inferior adhesion properties, polyesterfilms have the disadvantage that, despite their frequent use and widefield of applications, they charge up electrostatically to anundesirably high degree, like all plastics do.

In order to avoid this disadvantage it is known to add antistatic agentsto the plastic materials or to provide them with a thin surface coatingof an antistatic agent. When materials are treated according to thefirst-mentioned method, the antistatic effect which can be achieved isunsatisfactory or, if the added amount of antistatic agent is increased,their mechanical properties become clearly worse. When the materials areprovided with superficial coatings, a more efficient prevention ofelectrostatic surface charge-up is possible. In most cases, theantistatic agents used are water-soluble, low molecular-weight orpolymeric products. Thus, the applied coatings only possess a limitedresistance to moisture or water, respectively. Moreover, these coatingsare easily abraded mechanically, whereby their antistatic action islost, or they get tacky when the films are heated.

If a combination of customary antistatic agents and binders is used,antistatic coatings exhibiting improved mechanical strength areobtained, but their antistatic effect is lost when water is allowed toact upon the film.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved polyester film having an antistatic surface coating produced bytreatment of the finished film with an aqueous dispersion.

It is also an object of the invention to provide such a polyester filmwherein the dried coating is highly resistant to moisture and water.

Another object of the invention is to provide such a film which fully orat least substantially maintains its anti-electrostatic properties undermechanical strain.

Still another object of the invention is to provide a polyester filmhaving a surface that does not become tacky upon being heated.

It is also an object of the invention to provide an improved diazotypeduplicating film having an antistatic coating on its rear surface.

In accomplishing the foregoing objects, there has been providedaccording to one aspect of the present invention a biaxiallystretch-oriented and heat-set polyester film having, on at least onesurface, a coating comprising a copolyester containing sulfonate groups,wherein said coating comprises at least one salt derived from an aminoacid or a protein hydrolysate.

In accordance with another aspect of the present invention there hasbeen provided a diazotype duplicating film comprising a polyester film,as defined above, as the base material having the coating on the rearsurface of the film, and a diazotype reproduction layer on the frontsurface of the film.

Further objects, features and advantages of the invention will becomeapparent from the detailed description of preferred embodiments thatfollows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a biaxially stretchoriented and heat-setpolyester film which on at least one surface is provided with a coatingcomprising a copolyester that contains sulfonate groups. At least onesalt derived from amino acids or protein hydrolysates, in particular asalt of N-acylsarcosine or a salt of an Nacyl derivative of proteinhydrolysates is contained in the coating of the film. Preferably, thesalt is present in the coating in an amount of from about 5 to 25percent by weight, and particularly in an amount of from about 6 to 20percent by weight.

The present invention makes available polyester films provided with anantistatic coating, which can, for example, be used as layer supportsfor photosensitive or electrophotographic recording materials, as filmswhich can be marked, as mounting films, as cassette slip sheets or asvideo tape leader tapes, the antistatic effect of which is not lost dueto the action of moisture, water, mechanical strain or heat.

Polyester films provided with the antistatic coatings according to thisinvention are preferably employed for diazo type duplicating films orfor films on which an image can be produced by electrophotographicmeans, and which are processed at high temperatures. Even afterprocessing, a permanent antistatic effect is maintained.

At least one surface of the polyester film according to the presentinvention is provided with a coating including a binder which iscomprised of a copolyester that contains sulfonate groups and which isdispersible in water and in which at least one antistatic agent isembedded.

Substances which are commonly employed as surfactants have surprisinglyproved to be good antistatic agents. They can be derived from aminoacids. Nacylsarcosines, such as compounds of the fatty acid type, havinglauryl, cocoyl, stearyl or oleyl groups, in the form of their ammonium,alkali metal or, for example, triethanolamine salts, have been found tobe useful. The protein hydrolysates in the form of their salts, whichare obtained from natural collagen and which are condensed withlong-chain fatty acids, such as coconut fatty acid, are also suitable.

It is of great advantage that these substances are physiologically safeand are, for example, even employed in the cosmetics industry. Thus,users need not take any additional precautions to prevent environmentalpollution during the processing of the substances.

Other antistatic agents either showed a less durable antistatic actionor the amounts which had to be added were too high. In the case of someother antistatic agents, additional amounts of surfactants were requiredto achieve sufficient wetting of the polyester surface with the aqueoussolution.

The sulfonate group-containing copolyesters employed in accordance withthis invention are known, for example, from German Auslegeschrift No. 1816 163 (equivalent to U.S. Pat. No. 3,734,874), German Patent No. 28 13014 (equivalent to U.S. Pat. No. 4,252,885) or from U.S. Pat. No.3,563,942, the disclosures of which are hereby incorporated byreference. The polyesters are essentially soluble or dispersible inwater, and are obtained by reacting aliphatic, cycloaliphatic oraromatic dicarboxylic acids with aliphatic, cycloaliphatic or aromaticdiols, whereby the esterifiable dicarboxylic acids or diols carry an-S03M group on an aromatic ring, with M denoting a hydrogen or metalion.

To improve the thermal stability of the coating, the glass transitiontemperature of the copolyesters employed is preferably as high aspossible.

It has also been found advantageous to add crosslinking agents to thecoating. The crosslinking agents preferably are commercially availableformaldehyde compounds which are partially or completely etherified andderived from urea or melamine. Oligo-methoxymethylmelamines, such ashexamethoxymethylmelamine, have proved to be particularly usefulcompounds. The crosslinking agents are added to the coating solution insmall amounts. The coating solutions have to be rendered acidic toachieve a sufficiently rapid crosslinking reaction.

Other thermally stable binders which can be processed as aqueousdispersions may also be added to the system. For this purpose,polymethylmethacrylate plastic dispersions are used to particularadvantage. Particularly clear films are obtained when no second bindercomponent is required.

The aqueous solutions can be applied by means of any application systemwhich is able to produce sufficiently thin, homogeneous coatings. Thedried antistatic coatings have weights of from about 0.05 to 3.0 g/m²,depending on the particular use intended for the individual coatedpolyester film. The antistatic coatings can be applied as single-sidedor double-sided coatings. This also depends on the use intended for therespective material.

The polyester films employed preferably are made frompolyalkyleneterephthalate. This material is, in particular, used fordiazo duplicating films comprising a polyethyleneterephthalate filmsupport, the rear surface of which is provided with a permanentlyantistatic coating.

The coatings applied to the polyester films according to this inventionmay furthermore contain finely divided inorganic or organic substanceswhich serve, for example, as slip agents or matting agents. The coatingscan optionally be colored by means of dyes or pigments.

The invention will now be explained in greater detail by reference tothe Examples which follow.

The following, commercially available, anionic products are employed asantistatic agents: fatty acid sarcosinate (sodium salt) (1); palm nutfatty acid sarcosinate (2); lauroyl sarcosinate (3); palm nut/coconutfatty acid sarcosinate (4); a mixture of mono-, di- andtri-(alkyltetraglycol ether)-o-phosphoric acid esters (5); and a coconutfat condensation product with protein, as a potassium salt (6). Thesesubstances are available from Hoechst AG under the tradenames ARKOMON®,MEDIALAN® and HOSTAPHAT®, and from Chemische Fabrik Gruenau under thetradename LAMEPON®.

The employed sulfonate group-containing copolyesters, which are, forexample, available from Eastman Chemical Products Inc., arecharacterized by the following data:

    ______________________________________                                                Molecular            Hydroxyl                                                                             Acid                                      Name    Weight    Tg (°C.)                                                                          Number Number                                    ______________________________________                                        AQ 29   14,000    29         6.0    <2                                        AQ 38   18,000    38         5.0    <2                                        AQ 55   18,000    55         5.3    <2                                        ______________________________________                                    

EXAMPLE 1

A 100 μm thick, clear polyethylene terephthalate film, which had beenoriented by biaxial stretching and heat-set was used as the basematerial. Coating was performed with the aid of a wire wound doctor. Wetfilms having a thickness of 12 μm each were obtained from solutions ofthe following compositions: Basic solution, solids content 10%:

207 g of deionized water,

60 g of isopropyl alcohol, and

133 g of a 33% copolyester dispersion of AQ 55

Batches of this basic solution, which each weighed 25 g, were then mixedwith 3 g of 5% strength aqueous solutions of one of the above-describedantistatic agents.

The wet coatings were dried in a circulating air drying cabinet at 120°C. for 3 minutes. Clear, thin coatings were obtained, which were testedfor adhesion, water-resistance, tendency to become tacky and surfaceresistance. Table 1 shows the results obtained with the coatingscontaining 6% of the antistatic agent.

                  TABLE 1                                                         ______________________________________                                        Antistatic      Water       Tackiness                                                                            Surface Resi-                              Agent  Adhesion Resistance  Test   stance (ohm)                               ______________________________________                                        --     poor     good        poor   >10.sup.14                                 (1)    good     substantially                                                                             good   7.0 × 10.sup.10                                      good                                                          (6)    good     substantially                                                                             good   6.5 × 10.sup.11                                      good                                                          (2)    good     good        good   6.5 × 10.sup.10                      (4)    good     substantially                                                                             good   3.2 × 10.sup.10                                      good                                                          (5)    medium   substantially                                                                             good   5.5 × 10.sup.11                             to good  good                                                          (6)    good     substantially                                                                             good   9.0 × 10.sup.9                                       good                                                          ______________________________________                                    

It was found that smeary, unusable surfaces were obtained when theconcentration of (5) was increased.

Test procedures:

1. Adhesion of the Coating

The film is scratched by means of a criss-cross cut in accordance withDIN 53, 151. A strip of adhesive tape (TESTBAND® 104 colorless) ispressed onto the scratched surface areas with the aid of the handle of apair of scissors and is pulled off with a jerk. If the adhesion of thecoating is good, no portions thereof must stick to the adhesive tape.

2. Water resistance

A drop of deionized water is wiped over the surface to be tested. Awater-resistant coating will not be dissolved away.

3. Tackiness test

The film sample to be tested is introduced between a pair of heatedrollers (85° C.) of a blueprint machine. The test result is good if nomarkings are left on the antistatic coating.

4. Surface resistance

The samples are conditioned at about 23° C. and 50% relative humidityfor 24 hours. The surface resistance is determined by means of springreed electrodes in accordance with DIN 53,482. It is a measure for theantistatic action of the coating and should be less than to 1.0×1012 ohmif the antistatic effect is to be classified as being good.

EXAMPLE 2

In accordance with Example 1, antistatic coatings were prepared from thefollowing solutions:

I.

212 g of deionized water,

60 g of isopropanol,

133 g of a copolyester dispersion (30% strength) of AQ 29.

II.

185 g of deionized water,

60 g of isopropyl alcohol,

133 g of copolyester dispersion (25% strength) of AQ 38.

III.

212 g of deionized water,

60 g of isopropyl alcohol,

133 g of copolyester dispersion (30% strength) of AQ 55.

190 g of each of these solutions were mixed with 20 g of a 10% strengthsolution of a: (2) or b: (4). Thus, the coatings contained 10% of theantistatic admixture, which also acted as a surfactant. In all cases,clear coatings exhibiting excellent adhesion to the polyester film wereobtained. The test results are compiled in Table 2.

                  TABLE 2                                                         ______________________________________                                                 Water       Tackiness  Surface Resi-                                 Solution Resistance  Test       stance (ohm)                                  ______________________________________                                        Ia       substantially                                                                             substantially                                                                            2.7 × 10.sup.10                                  good        good                                                     Ib       substantially                                                                             good       6.5 × 10.sup.10                                  good                                                                 IIa      good        medium     2.3 × 10.sup.10                         IIb      substantially                                                                             substantially                                                                            5.0 × 10.sup.9                                   good        good                                                     IIIa     good        good       8.0 × 10.sup.9                          IIIb     substantially                                                                             substantially                                                                            1.3 × 10.sup.10                                  good        good                                                     ______________________________________                                    

EXAMPLE 3

Antistatic coatings were prepared as described in Example 1. SolutionsIa to IIIb according to Example 2 additionally comprisedhexamethoxymethylmelamine (CYMEL® 300) or a partly etherifiedmelamine/formaldehyde resin (MADURIT® MW 815) as crosslinking agents.The dried antistatic coating had a content of crosslinking agent of 5%.

Prior to being processed, the solutions were adjusted to a pH value ofabout 4.3 by means of citric acid.

After driving, clear coatings were obtained which adhered excellently tothe polyester film. Further test results are summarized in Table 3below.

                  TABLE 3                                                         ______________________________________                                              Crosslinking                                                                             Water     Tackiness                                                                              Surface Resi-                             Solution                                                                            Agent      Resistance                                                                              Test     stance (ohm)                              ______________________________________                                        Ia    CYMEL      subst. good                                                                             good     1.1 × 10.sup.11                           MADURIT    good      good     1.5 × 10.sup.12                     Ib    CYMEL      good      good     3.2 × 10.sup.11                           MADURIT    good      good     2.9 × 10.sup.12                     IIa   CYMEL      good      subst. good                                                                            3.8 × 10.sup.10                           MADURIT    good      good     7.5 × 10.sup.11                     IIb   CYMEL      good      good     1.3 × 10.sup.11                           MADURIT    good      good     1.6 × 10.sup.12                     IIIa  CYMEL      subst. good                                                                             good     2.2 × 10.sup.11                           MADURIT    good      good     2.6 × 10.sup.12                     IIIb  CYMEL      medium    good     2.8 × 10.sup.11                           MADURIT    good      good     9.0 × 10.sup.12                     ______________________________________                                    

EXAMPLE 4

In the same manner as described in Example 1, antistatic coatings wereprepared from solutions having the following compositions:

Basic solution, solids content 10%:

233 g of deionized water,

60 g of isopropyl alcohol,

67 g of copolyester dispersion (30% strength) of AQ 55,

40 g of 50% strength dispersion of polymethyl

methacrylate.

25 g batches of this basic solution were each mixed with 3 g of a 5%strength aqueous solution of an antistatic agent. The resulting driedcoatings showed good adhesion to the polyester film. They had a contentof antistatic agent of 6%. The test results are shown in Table 4 below.

                  TABLE 4                                                         ______________________________________                                        Antistatic                                                                              Water        Tackiness                                                                              Surface Resi-                                 Agent     Resistance   Test     stance (ohm)                                  ______________________________________                                        none      good         poor     >10.sup.14                                    (1)       substantially                                                                              good     3.2 × 10.sup.10                                   good                                                                (6)       substantially                                                                              good     4.6 × 10.sup.11                                   good                                                                (2)       good         good     1.0 × 10.sup.10                         (4)       substantially                                                                              good     1.1 × 10.sup.10                                   good                                                                ______________________________________                                    

EXAMPLE 5

100 μm thick film was coated with antistatic coatings as described inExample 1. Stock solutions II and III of Example 2 were employed asbasic solutions.

These solutions were mixed with 10% strength solutions of a: (2) and b:(4). The coatings were adjusted to contents of antistatic agents of 6%,10% and 14%. The coatings thus obtained were clear and adhered well tothe film. The test results are summarized in Tables 5.1 and 5.2 below.

                  TABLE 5.1                                                       ______________________________________                                        Solution II with copolyester dispersion AQ 38                                 Anti- Added    Water           Surface Resistance                             static                                                                              Amount   Resis-  Tackiness                                                                             (ohm)                                          Agent (%)      tance   Test    A       B.sup.+                                ______________________________________                                        (2)    0       good    poor    >10.sup.14                                                                            --                                            6       good    subst. good                                                                           1.2 × 10.sup.10                                                                 1.3 × 10.sup.11                        10       good    good    7.0 × 10.sup.9                                                                  1.5 × 10.sup.11                        14       good    good    7.5 × 10.sup.9                                                                  1.0 × 10.sup.11                  (4)    6       good    good    1.4 × 10.sup.10                                                                 1.7 × 10.sup.11                        10       good    good    7.5 × 10.sup.9                                                                  6.5 × 10.sup.10                        14       good    good    4.2 × 10.sup.9                                                                  2.8 × 10.sup.10                  ______________________________________                                    

                  TABLE 5.2                                                       ______________________________________                                        Solution III with copolyester dispersion AQ 55                                Anti  Added              Tacki-                                                                              Surface Resistance                             static                                                                              Amount   Water     ness  (ohm)                                          Agent (%)      Resistance                                                                              Test  A       B.sup.+                                ______________________________________                                        (2)   0        good      good  >10.sup.14                                                                            --                                           6        good      good  4.0 × 10.sup.10                                                                 7.5 × 10.sup.11                        10       good      good  4.2 × 10.sup.10                                                                 1.9 × 10.sup.11                        14       good      good  1.5 × 10.sup.10                                                                 2.7 × 10.sup.11                  (4)   6        good      good  3.6 × 10.sup.10                                                                 1.0 × 10.sup.11                        10       good      good  5.0 × 10.sup.9                                                                  9.0 × 10.sup.10                        14       subst. good                                                                             good  1.5 × 10.sup.10                                                                 6.5 × 10.sup.10                  ______________________________________                                         .sup.+ The surface resistance value B was measured after treatment of the     sample with water.                                                       

To determine the surface resistance value after treatment with water,the samples were immersed into deionized water, then dried at roomtemperature and conditioned at a temperature of about 23° C. and 50%relative humidity for 24 hours. Thereafter, the surface resistance wasmeasured as described in Example 1. Low differences between the originalvalues and the values determined in this test indicate good waterresistance of the antistatic coatings.

EXAMPLE 6

The rear surfaces of diazotype duplicating films comprising 125 μm thickpolyethyleneterephthalate film as the base material were provided, in acontinuously working coating device, with thin coatings of the followingsolutions:

Solution I:

118.7 g of copolyester dispersion (30% strength) of AQ 55,

189.7 g of deionized water,

53.6 g of isopropyl alcohol,

38.0 g of (2), 10 % solution in water.

Solution II:

100.0 g of copolyester dispersion (30% strength) of AQ 55,

160.0 g of deionized water,

45.2 g of isopropyl alcohol,

32.0 g of (2), 10% strength solution in water,

36.6 g of hexamethoxymethylmelamine, 5% strength, in ethyl alcohol/water(1:1),

26.5 g of citric acid, 5% strength, in water.

Drying was performed within about 20 seconds by means of stronglycirculating air at 120° C. The properties of the resulting coatings arecompiled in Table 6 below:

                  TABLE 6                                                         ______________________________________                                                                  Tacki-                                                                              Surface Resistance                            Solution                                                                              Ad-     Water     ness  (ohm)                                         Employed                                                                              hesion  Resistance                                                                              Test  A      B                                      ______________________________________                                        none    --      --        good  >10.sup.14                                    I       good    subst. good                                                                             good  2.8 × 10.sup.9                                                                 4.8 × 10.sup.10                  II      good    subst. good                                                                             good  1.9 × 10.sup.9                                                                 2.0 × 10.sup.10                  ______________________________________                                    

These three materials were furthermore evaluated with respect of theirease of handling by piling them up in stacks comprising sheets of 148mm×105 mm.

The individual sheets must not slip too much, and it must be possible tostack and destack them without difficulty. The sample prepared usingsolution II had the best result.

EXAMPLE 7

The rear surfaces of diazotype duplicating films comprising 125 μm thickpolyethyleneterephthalate film as the base material were provided, in acontinuously working coating device, with thin coatings of the followingsolutions:

Solution I:

100.2 g of copolyester dispersion (30% strength) of AQ 55,

134.3 g of deionized water,

53.7 g of isopropyl alcohol,

79.2 g of (4), 10% strength solution in water,

6.2 g of hexamethoxymethylmelamine, 5% strength, in ethyl alcohol/water(1:1),

26.4 g of citric acid, 5% strength, in water.

Solution II:

100.8 g of copolyester dispersion (30% strength), of AQ 55,

136.8 g of deionized water,

53.7 g of isopropyl alcohol,

79.2 g of (4), 10% strength solution in water,

3.1 hexamethoxymethylmelamine, 5% strength, in ethyl alcohol/water(1:1),

26.4 g of citric acid, 5 % strength, in water.

Drying was performed within about 20 seconds by means of stronglycirculating air at 120° C. The properties of the resulting coatings arecompiled in Table 7 below:

                  TABLE 7                                                         ______________________________________                                                                  Tacki-                                                                              Surface Resistance                            Solution                                                                              Ad-     Water     ness  (ohm)                                         Employed                                                                              hesion  Resistance                                                                              Test  A      B                                      ______________________________________                                        none    --      --        good  >10.sup.14                                    I       good    subst. good                                                                             good  5.5 × 10.sup.9                                                                 3.2 × 10.sup.10                  II      good    subst. good                                                                             good  6.5 × 10.sup.9                                                                 3.4 × 10.sup.10                  ______________________________________                                    

The handling test, performed with sheets of 148 mm×105 mm, in accordancewith the description given in Example 6, gave good results in all threecases.

What is claimed is:
 1. A diazotype duplicating film comprising:a base material comprising a biaxially stretch-oriented and heat-set polyester film; a coating on a first surface of the polyester film comprising a copolyester, containing sulfonate groups, and an antistatically effective amount of at least one salt derived from an amino acid or a protein hydrolysate; and a diazotype reproduction layer on the opposite surface of the film.
 2. A diazotype duplicating film as claimed in claim 1, wherein the copolyester coating contains about 5% to 25% by weight of the salt.
 3. A diazotype duplicating film as claimed in claim 1, further comprising a crosslinking agent contained in the coating.
 4. A diazotype duplicating film as claimed in claim 1, wherein the copolyester coating further comprises a polymethylmethacrylate.
 5. A diazotype duplicating film as claimed in claim 1, wherein the polyester base material comprises a polyethyleneterephthalate.
 6. A diazotype duplicating film as claimed in claim 1, wherein said salt is an ammonium, alkali metal or triethanolamine salt.
 7. A diazotype duplicating film as claimed in claim 1, wherein the salt comprises a salt of an N-acylsarcosine.
 8. A diazotype duplicating film as claimed in claim 7, wherein the acyl group of the N-acylsarconsine is a lauryl, cocoyl, stearyl or oleyl group.
 9. A diazotype duplicating film as claimed in claim 1, wherein the salt comprises a salt of an N-acyl derivative of a protein hydrolysate.
 10. A diazotype duplicating film as claimed in claim 9, wherein said protein hydrolysate is obtained from natural collagen and is condensed with a fatty acid.
 11. A diazotype duplicating film as claimed in claim 10, wherein said fatty acid is coconut fatty acid.
 12. A diazotype duplicating film as claimed in claim 1, wherein the crosslinking agent comprises at least one partly or completely etherified formaldehyde compound derived from urea or melamine.
 13. A diazotype duplicating film as claimed in claim 12, wherein the crosslinking agent comprises hexamethoxymethylmelamine.
 14. A process for producing a diazotype duplicating film as claimed in claim 1, comprising the steps of:applying to a first surface of a biaxially-stretched and heat-set polyester film a coating comprising a copolyester, containing sulfonate groups, and an antistatically effective amount of at least one salt derived from an amino acid or a protein hydrolysate; drying said coating; and applying to an opposite surface of the polyester film a diazotype reproduction layer.
 15. A process as claimed in claim 14, wherein said coating contains about 5% to 25% by weight of said salt. 